1. Field of the Invention
The present invention is related to bipolar plates with improved hydrophilicity for fuel cell applications.
2. Background
Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”) to provide ion transport between the anode and cathode.
In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Each electrode has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote oxidation of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water, which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”) which, in turn, are sandwiched between a pair of non-porous, electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cell stacks in order to provide high levels of electrical power.
The electrically conductive plates currently used in fuel cells provide a number of opportunities for improving fuel cell performance. For example, it is desirable to minimize the agglomeration of water droplets within flow channels in the plates. To this end, fuel cells are typically coated with a hydrophilic coating. Currently, hydrophylic layers are applied to a conductive plate via a multilayer adsorption (MLA) process. Typically, such processes require 4 dip cycles (i.e., 4 bilayers, 1 bilayer consists of a layer of silica on top of a layer of a cationic polymer) in a hydrophilic coating such as silica-based NanoX. Although such processes work reasonably well, MLA methods are undesirably labor intensive often taking up to 40 minutes to complete.
Although recent stack data indicate that a superhydrophilic coating is not necessary in the active area of Au-coated stainless steel bipolar plates to pass low power stability (LPS), future plate designs and system operating conditions may require such a coating for water management. Presently, silica-based hydrophilic coatings (e.g., EMS, NanoX) applied using a multilayer adsorption (MLA) process (includes use of a cationic polymer) are not sufficiently water stable. In stacks S0340 (3500 hrs) and S0949 (5100 hrs), the silica-coated plates became grossly non-wicking (less hydrophilic) after fuel cell exposure due to silica and cationic polymer dissolution. A more hydrolytically stable material is needed to replace the water-soluble cationic polymer.
Accordingly, there is a need for improved methodology for applying hydrophilic coatings at the surfaces of bipolar plates used in fuel cell applications.